invalid switch errors

Unanswered Question
Mar 24th, 2009

I keep getting "packet with invalid source mac addr message" on one of my switches, on its

portchannel interface..i checked the mac addr which the message gives out.

But it couldnt be traced..doesnt show up in the mac/arp table..even tried looking for it onto the

standby switch.

Any ideas how to trace source of this mac-addr.


I have this problem too.
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Giuseppe Larosa Wed, 03/25/2009 - 02:03

Hello Sunny,

this is quite common:

the invalid source MAC address can be:

an all zeroes (typical of just initialized not configured VMware VM or IPv6 stacks)

a multicast MAC address

note: are multicast MAC address those that have set the 1 of righmost bit of first byte like:

0100. (multicast)

other bits can be set

if the MAC is not regular unicast you cannot trace it is not learned and not shown in the CAM tables

We get these messages from time to time but no real issues.

Hope to help


tom_fox_ki4qqq Wed, 03/25/2009 - 04:11


I believe multicast MAC addresses start with an odd number (01, 03, etc), not just 01. I found a doc for another thread regarding Microsoft NLB, and it said to configure the Microsoft cluster to use a multicast MAC beginning with 03, since starting with 01 may interfere with IGMP.

Personally, I never heard of that before, but after digging around, I guess it's possible.


suthomas1 Wed, 03/25/2009 - 07:34

Thanks Giuseppe!

0900.8F74.1B31..this is the one i see.But may be am naive in this question..if this is not regular unicast & is a multicast(if am correct) do these get generated & reflected on to the devices.

Just looking for a logical explaination.

Thanks a ton!!

Giuseppe Larosa Wed, 03/25/2009 - 07:46

Hello Sunny, Tom,

I was meaning that the bit 0 of first byte has to be set so yes odd values are treated as multicast and cannot be legitimate source address

so 0900.8F74.1B31 this is a multicast because

09 = 0000 1001 <= the bit is set

Hope to help


suthomas1 Wed, 03/25/2009 - 08:19

That was a gud explaination...thanks for it.

One more thing...any possible reasons/sources for this multicast mac to come up.



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